The primary objective of this work is to understand the molecular details of DNA replication with particular emphasis on the problems of initiation of DNA chains, and the nature of the swivel. We are currently studying the DNA "untwistase", an enzyme which by virtue of the fact that it introduces a transient nick into double-strained DNA, may possibly have the role of providing a swivel in vivo. We propose to characterize and purify the activity and study its mechanism of action. We have succeeded in using E. coli RNA polymerase to synthesize a primer for DNA synthesis on closed circular superhelical SV40 DNA. In this system DNA synthesis is limited, presumably owing to the topological constraints imposed by the fact that the template DNA strands are unnicked. We propose to use this system to test whether the purified DNA "untwistase" can provide a swivel for DNA replication in vitro.